Most current wireless communication systems are composed of nodes configured with a single transmit and receive antenna. However, for a wide range of wireless communication systems, it has been predicted that the performance, including capacity, may be substantially improved through the use of multiple transmit and/or multiple receive antennas. Such configurations form the basis of “smart” antenna techniques. Smart antenna techniques, coupled with space-time signal processing, can be utilized both to combat the deleterious effects of multipath fading of a desired incoming signal and to suppress interfering signals. In this way, both the performance and capacity of digital wireless systems in existence or being deployed (e.g., CDMA-based systems, TDMA-based systems, WLAN systems, and OFDM-based systems such as IEEE 802.11a/g) may be improved.
At least some of the impairments to the performance of wireless systems of the type described above may be at least partially ameliorated by using multi-element antenna systems designed to introduce a diversity gain and suppress interference within the signal reception process. This has been described, for example, in “The Impact of Antenna Diversity On the Capacity of Wireless Communication Systems”, by J. H. Winters et al., IEEE Transactions on Communications, vol. 42, no. 2/3/4, pages 1740-1751, February 1994. Such diversity gains improve system performance by mitigating multipath for more uniform coverage, increasing received signal-to-noise ratio for greater range or reduced required transmit power, and providing more robustness against interference or permitting greater frequency reuse for higher capacity.
Within communication systems incorporating multi-antenna receivers, a set of M receive antennas may be capable of nulling up to M−1 interferers. Accordingly, N signals may be simultaneously transmitted in the same bandwidth using N transmit antennas, with the transmitted signal then being separated into N respective signals by way of a set of N antennas deployed at the receiver. Systems of this type are generally referred to as multiple-input-multiple-output (MIMO) systems, and have been studied extensively. See, e.g., “Optimum combining for indoor radio systems with multiple users”, by J. H. Winters, IEEE Transactions on Communications, vol. COM-35, no. 11, November 1987; “Capacity of Multi-Antenna Array Systems In Indoor Wireless Environment”, by C. Chuah et al., Proceedings of Globecom '98 Sydney, Australia, IEEE 1998, pages 1894-1899 November 1998; and “Fading Correlation and Its Effect on the Capacity of Multi-Element Antenna Systems” by D. Shiu et al., IEEE Transactions on Communications, vol. 48, no. 3, pages 502-513, March 2000.
Some multi-element antenna arrangements (e.g., some MIMOs) provide system capacity enhancements that can be achieved using the above-referenced configurations. Under the assumption of perfect estimates of the applicable channel at the receiver, in a MIMO system the received signal decomposes to M “spatially-multiplexed” independent channels. This results in an M-fold capacity increase relative to single-antenna systems. For a fixed overall transmitted power, the capacity offered by MIMOs scales linearly with the number of antenna elements. Specifically, it has been shown that with N transmit and N receive antennas an N-fold increase in the data rate over a single antenna system can be achieved without any increase in the total bandwidth or total transmit power. See, e.g., “On Limits of Wireless Communications in a Fading Environment When Using Multiple Antennas”, by G. J. Foschini et al., Wireless Personal Communications, Kluwer Academic Publishers, vol. 6, no. 3, pages 311-335, March 1998. In experimental MIMO systems predicated upon N-fold spatial multiplexing, more than N antennas are often deployed at a given transmitter or receiver. This is because each additional antenna adds to the diversity gain and antenna gain and interference suppression applicable to all N spatially-multiplexed signals. See, e.g., “Simplified processing for high spectral efficiency wireless communication employing multi-element arrays”, by G. J. Foschini et al., IEEE Journal on Selected Areas in Communications, vol. 17, issue 11, November 1999, pages 1841-1852.
Although increasing the number of transmit and/or receive antennas enhances various aspects of the performance of MIMO systems, the provision of a separate RF chain for each transmit and receive antenna increases costs. Each RF chain is generally comprised a low noise amplifier, filter, downconverter, and analog-to-digital converter (A/D), with the latter three devices typically being responsible for most of the cost of the RF chain. In certain existing single-antenna wireless receivers, the single required RF chain may account for in excess of 30% of the receiver's total cost. It is thus apparent that as the number of transmit and receive antennas increases, overall system cost and power consumption may dramatically increase.
Another potentially attractive technique used, for example, to increase the data rate of a wireless link without necessarily increasing the required number of RF chains is channel bonding. In some applications, channel bonding may fall within a category of multiple access techniques, for example, employed in wireless communication systems. The channel bonding allocates and shares the available radio resource among various users. The available radio resource is decomposed into distinct logical channels in the time, frequency and code domains. A particular logical channel is defined by a frequency slot number, a time slot number and/or a code number in accordance with such decomposition. Typically (e.g., without channel bonding), a user is assigned one logical channel (e.g., a single frequency band over which a transmitter and receiver communicate). On the other hand, when channel bonding is used, a larger pool of bandwidth is made available for each user by permitting the user to communicate over multiple logical channels in parallel, thereby achieving higher data rates. An example of channel bonding in a system operating in accordance with a Time Division Multiple Access (TDMA) protocol is described in, for example, “EDGE: Enhanced Data Rates for GSM and TDMA/136 Evolution,” by A. Furuskar et al., IEEE Personal Communications Magazine, vol. 6, issue 3, pages 56-66 (June 1999). In the described EDGE system, larger numbers of time division slots are allocated to particular users to permit the users to experience greater throughput. Similarly, the use of channel bonding used in Code Division Multiple Access (CDMA), Frequency Division Multiple Access (FDMA) and TDMA-based systems is described in, for example, U.S. Patent Application Publication Nos. 20020197998 and 20020051435.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of ordinary skill in the art through comparison of such systems with some embodiments according to some aspects of the present invention as set forth in the remainder of the present application with reference to the drawings.